This disclosure relates generally to optical connectivity and more particularly to ferrules for fiber optic connectors, fiber optic assemblies incorporating ferrules, and methods for fabricating ferrules and fiber optic assemblies incorporating ferrules.
Optical fibers are useful in a wide variety of applications, including the telecommunications industry for voice, video, and data transmissions. In a telecommunications system that uses optical fibers, there are typically many locations where fiber optic cables carrying the optical fibers connect to equipment or other fiber optic cables. To conveniently provide these connections, fiber optic connectors are often provided on the ends of fiber optic cables. The process of terminating individual optical fibers from a fiber optic cable is referred to as “connectorization.” Connectorization can be done in a factory, resulting in a “pre-connectorized ” or “pre-terminated” fiber optic cable, or the field (e.g., using a “field-installable” fiber optic connector).
Regardless of where installation occurs, a fiber optic connector typically includes a ferrule with one or more bores that receive one or more optical fibers. The ferrule supports and positions the optical fiber(s) with respect to a housing of the fiber optic connector. Thus, when the housing of the fiber optic connector is mated with another connector (e.g., in an adapter), an optical fiber in the ferrule is positioned in a known, fixed location relative to the housing. This allows an optical connection to be established when the optical fiber is aligned with another optical fiber provided in the mating connector. For fiber optic connectors including one or more optical fibers that extend to a front face of the ferrule, there is typically physical contact between the front faces of the mating ferrules (and typically the ends of the mating fibers) to ensure that the optical connection can be established.
As optical networks march toward 400G Ethernet, optical transceiver speeds of 25 Gb/s are becoming commonplace with four level pulse amplitude modulation (PAM4), and higher data rates are anticipated in the future. The growing complexity and speed of transceivers elevate the importance of reducing insertion loss for optical connectors. Despite this, optical connectivity remains more expensive than copper connectivity, and reducing optical connector cost is essential for optical communications to penetrate deeper into short distance applications.
Table 1 shows the connector insertion loss grades defined by International Electrotechnical Commission (IEC) standard 61753-1, Edition 1.0: 2007-03 (Fibre optic interconnecting devices and passive components performance standard—Part 1: General and guidance for performance standards). The loss numbers specified in the foregoing standard are based on random mate (or each-to-each) measurement as defined by IEC 61300-3-34, Edition 3.0: 2009-01 (Examinations and measurements—Attenuation of random mated connectors). Random mate values are closer to practical operating conditions than typical values measured by reference jumpers.
TABLE 1AttenuationGradeRandom mate attenuationGrade A (TBD)≤0.07 dB mean≤0.15 dB max. for >97% of samplesGrade B≤0.12 dB mean≤0.25 dB max. for >97% of samplesGrade C≤0.25 dB mean≤0.50 dB max. for >97% of samplesGrade D≤0.50 dB mean≤1.0 dB max. for >97% of samples
Conventional single fiber connectors employ a precision ceramic ferrule as a means of supporting and aligning the optical fiber for obtaining low connection loss. The ferrule has tight tolerances in outer diameter, inner diameter, and concentricity. The ferrule, which typically is made of zirconia ceramics, has a diameter of 1.25 mm for LC connectors and a diameter of 2.5 mm for SC, ST, and FC connectors. An optical fiber is inserted into a micro-hole of a ferrule with a bonding agent such as epoxy. The optical fiber bonded to the ferrule undergoes cleaving, and multiple steps of polishing are applied to a fiber-ferrule assembly to obtain an end face geometry that meets requirements for a desired type of physical contact. Exemplary physical contact geometries include, but are not limited to, physical contact (PC), angled physical contact (APC), and ultra-physical contact (UPC) geometries.
Connector random mating insertion loss is determined by the offset of the center of the core from the center of the ferrule (e.g., a geometric center based on an outer surface of the ferrule), also known as core to ferrule eccentricity (CTFE), of the finished connector. Thus, CTFE represents the core to ferrule concentricity error. In conventional ferrule-based connectors, CTFE is affected primarily by: a) the concentricity between the micro-hole of the ferrule and the ferrule itself (e.g., the outer profile of the ferrule); b) the concentricity between the optical fiber and the micro-hole; and c) the concentricity between the core of the optical fiber and the cladding layer of the optical fiber. The fit between the outer diameter (OD) of the optical fiber and the inner diameter (ID) of the ferrule (as defined by the micro-hole) is very critical for low connector insertion loss. If the fiber OD is larger than the ferrule ID, or is simply too close in size to the ferrule ID, then the fiber cannot be inserted into the ferrule. Conversely, if the difference between the ferrule ID and fiber OD is too large, then such size difference will cause a wide distribution in CTFE and result in high connector insertion loss. A ferrule outer diameter and inner diameter typically have a standard deviation of 0.16 μm, while the eccentricity of a typical ferrule follows a Rayleigh distribution with a mode of 0.24 μm. The standard deviation of the outer diameter of a typical optical fiber is 0.16 μm, and the core to clad eccentricity follows a Rayleigh distribution with a mode of 0.13 μm.
FIG. 1 is a schematic cross-sectional view of a fiber core 11 within a bare optical fiber 4 that is arranged within a small diameter bore section 20 (or “micro-hole”) of a ferrule 12 having a substantially cylindrical shape. The bare optical fiber 4 has an outer diameter (OD) 6 that is smaller than an inner diameter (ID) 8 of the micro-hole 20. Three vectors r1, r2, and r3 represent three potential sources of variability in CTFE of a conventional fiber-ferrule assembly. The first vector r1 represents variability in CTFE due to non-concentricity of the micro-hole 20 relative to the cylindrical shape of the ferrule 12. The second vector r2 represents variability in CTFE due to difference between the OD 6 of the bare optical fiber 4 and the ID 8 of the micro-hole 20 (restated, non-concentricity of the cylindrical bare optical fiber 4 relative to the cylindrical shape of the micro-hole 20, which defines the ID 8). The third vector r3 represents variability in CTFE due to non-concentricity of the fiber core 11 relative to the cylindrical shape of the bare optical fiber 4 (which defines the OD 6).
Various solutions have been proposed to improve the precision of the fiber-to-ferrule fit and cancel the eccentricities by orienting the connector. For instance, the fibers and ferrules can be pre-selected into sub-populations according to the OD and ID. Connectors are made with closely matched sub-populations of fibers and ferrules. Another proposed solution involves local expansion of a fiber by a high temperature energy source to create a tight fit into a ferrule. Still another proposed solution involves preassembly of a fiber stub and a ferrule with matched OD and ID. Thereafter, a ferrule stub assembly is fusion spliced to at least one fiber in a cable assembly.
Conventional methods that rely on ferrule-to-fiber matching, precisely controlling ferrule geometries, and/or precisely controlling a fiber-ferrule bonding process to reduce CTFE and connector insertion loss entail complicated manufacturing processes. The industry continues to seek methods for improving CTFE and reducing connector insertion loss, but with reduced manufacturing complexity and cost.